Add various doc updates

They are still very far from complete.

docs/concentric-circles.md

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+Let's consider a bullseye with the following concentric circles:
+
+- Ring 0: small, simple systems
+  - Single jurisdiction
+  - Email + password
+  - A few roles
+  - Naïve or soft deletion
+  - Minimal audit needs
+
+- Ring 1: grown-up systems
+  - Long-lived data
+  - Changing requirements
+  - Shared accounts
+  - GDPR-style erasure/anonymization
+  - Some cross-border concerns
+  - Historical data must remain usable
+  - “Oops, we should have thought about that” moments
+
+- Ring 2: heavy compliance
+  - Formal audit trails
+  - Legal hold
+  - Non-repudiation
+  - Regulatory reporting
+  - Strong identity guarantees
+  - Jurisdiction-aware data partitioning
+
+- Ring 3: banking / defense / healthcare at scale
+  - Cryptographic auditability
+  - Append-only ledgers
+  - Explicit legal models
+  - Independent compliance teams
+  - Lawyers embedded in engineeRing
+
+diachron is designed to be suitable for Rings 0 and 1.  Occasionally we may
+look over the fence into Ring 2, but it's not what we've principally designed
+for.  Please take this framing into account when evaluating diachron for
+greenfield projects.